The alternative strategy is to form the mercaptan before introducing the N-oxide moiety. 2-Mercaptopyridine was originally synthesized in 1931 by heating 2-chloropyridine with calcium bisulfide,[6] an approach similar that first used to prepare pyrithione.[8] The analogous thiourea approach via a uronium salt was reported in 1958 and provides a more convenient route to 2-mercaptopyridine.[7] Oxidation to the N-oxide can then be undertaken.

Tautomerisation of the sodium salt of pyrithione(thione form on the left, thiolate form on the right)

Pyrithione exists as a pair of prototropes, a form of tautomerism whereby the rapid interconversion of constitutional isomers involves the shift of a single proton, in this case between the sulfur and oxygen atoms (shown in the infobox).[3][22][23]Salts of the conjugate base of pyrithione can also be considered to exhibit tautomerism by notionally associating the sodium ion with whichever heteroatom bears the negative charge of the anion (as opposed to the formal charges associated with the N-oxide); however, considering the anion alone, this could also be described as an example of resonance.

Structures of 1:2 complexes of zinc and the conjugate base of pyrithioneTop: Structural formula of the monomerBottom: Ball-and-stick model of the dimer

The conjugate base of pyrithione is an anion containing two donor atoms, a sulfur atom and an oxygen atom each bearing a negative formal charge; the nitrogen atom remains formally positively charged. The thiolate anion can be formed by reaction with sodium carbonate, and zinc pyrithione is formed when zinc chloride is added.[10] The anion can act as either a monodentate or bidentateligand and forms a 1:2 complex with a zinc(II) metal centre. Zinc pyrithione has been used since the 1930s though its preparation was not disclosed until a 1955 British patent[13] in which pyrithione was reacted directly with hydrated zinc sulfate in ethanol.[9] In its monomeric form, zinc pyrithione has two of the anions chelated to a zinc centre with a tetrahedral geometry. In the solid state, it forms a dimer in which each zinc centre adopts a trigonal bipyramidal geometry with two of the anions acting as bridging ligands coordinated through the oxygen atoms in the axial positions.[26] In solution, the dimers dissociate via scission of zinc-oxygen bonds to each bridging ligand. Further dissociation of the monomer into its constituents can occur and is undesirable as the complex is more potent in medical applications; for this reason, zinc carbonate can be added to formulations as it inhibits the monomer dissociation.[27]

1.
Melting point
–
The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure. At the melting point the solid and liquid phase exist in equilibrium, the melting point of a substance depends on pressure and is usually specified at standard pressure. When considered as the temperature of the change from liquid to solid. Because of the ability of some substances to supercool, the point is not considered as a characteristic property of a substance. For most substances, melting and freezing points are approximately equal, for example, the melting point and freezing point of mercury is 234.32 kelvins. However, certain substances possess differing solid-liquid transition temperatures, for example, agar melts at 85 °C and solidifies from 31 °C to 40 °C, such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is close to 0 °C. In the presence of nucleating substances the freezing point of water is the same as the melting point, the chemical element with the highest melting point is tungsten, at 3687 K, this property makes tungsten excellent for use as filaments in light bulbs. Many laboratory techniques exist for the determination of melting points, a Kofler bench is a metal strip with a temperature gradient. Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point, differential scanning calorimetry gives information on melting point together with its enthalpy of fusion. A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window, the several grains of a solid are placed in a thin glass tube and partially immersed in the oil bath. The oil bath is heated and with the aid of the melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, the measurement can also be made continuously with an operating process. For instance, oil refineries measure the point of diesel fuel online, meaning that the sample is taken from the process. This allows for more frequent measurements as the sample does not have to be manually collected, for refractory materials the extremely high melting point may be determined by heating the material in a black body furnace and measuring the black-body temperature with an optical pyrometer. For the highest melting materials, this may require extrapolation by several hundred degrees, the spectral radiance from an incandescent body is known to be a function of its temperature. An optical pyrometer matches the radiance of a body under study to the radiance of a source that has been previously calibrated as a function of temperature, in this way, the measurement of the absolute magnitude of the intensity of radiation is unnecessary. However, known temperatures must be used to determine the calibration of the pyrometer, for temperatures above the calibration range of the source, an extrapolation technique must be employed

2.
Shampoo
–
Shampoo is a hair care product, typically in the form of a viscous liquid, that is used for cleaning hair. Less commonly, shampoo is available in bar form, like a bar of soap, Shampoo is used by applying it to wet hair, massaging the product into the hair, and then rinsing it out. Some users may follow a shampooing with the use of hair conditioner, the goal of using shampoo is to remove the unwanted build-up in the hair without stripping out so much sebum as to make hair unmanageable. Shampoo is generally made by combining a surfactant, most often sodium lauryl sulfate or sodium sulfate, with a co-surfactant. There are also intended for animals that may contain insecticides or other medications to treat skin conditions or parasite infestations such as fleas. The word shampoo entered the English language from India during the colonial era and it dates to 1762, and is derived from Hindi chāmpo, itself derived from the Sanskrit root chapayati. Sake Dean Mahomed is identified as a promoter of the practice in Britain. In India, a variety of herbs and their extracts were used as shampoos since ancient times, a very effective early shampoo was made by boiling Sapindus with dried Indian gooseberry and a few other herbs, using the strained extract. Sapindus, also known as soapberries or soapnuts, is called Ksuna in ancient Indian texts and its fruit pulp contains saponins which are a natural surfactant, the extract of soapberries, a tropical tree widespread in India, creates a lather which Indian texts called phenaka. It leaves the hair soft, shiny and manageable, other products used for hair cleansing were shikakai, soapnuts, hibiscus flowers, ritha and arappu. Guru Nanak, the prophet and the first Guru of Sikhism, made references to soapberry tree. Cleansing with hair and body massage during daily strip wash was an indulgence of early colonial traders in India, when they returned to Europe, they introduced the newly learnt habits, including hair treatment they called shampoo. During the early stages of shampoo in Europe, English hair stylists boiled shaved soap in water and added herbs to give the hair shine, commercially made shampoo was available from the turn of the 20th century. A1914 ad for Canthrox Shampoo in American Magazine showed young women at camp washing their hair with Canthrox in a lake, magazine ads in 1914 by Rexall featured Harmony Hair Beautifier and Shampoo. In 1927, liquid shampoo was invented by German inventor Hans Schwarzkopf in Berlin, originally, soap and shampoo were very similar products, both containing the same naturally derived surfactants, a type of detergent. Modern shampoo as it is today was first introduced in the 1930s with Drene. Early shampoos used in Indonesia were made from the husk and straw of rice, the husks and straws were burned into ash, and the ashes are mixed with water to form lather. The ashes and lather were scrubbed into the hair and rinsed out, leaving the hair clean, afterwards, coconut oil was applied to the hair in order to moisturize it

3.
Fouling
–
Fouling is the accumulation of unwanted material on solid surfaces to the detriment of function. The fouling materials can consist of living organisms or a non-living substance. Other terms used in the literature to describe fouling include, deposit formation, encrustation, crudding, deposition, scaling, scale formation, slagging, and sludge formation. This article is devoted to the fouling of industrial heat exchangers. In the cooling technology and other fields, a distinction is made between macro fouling and micro fouling. Of the two, micro fouling is the one which is more difficult to prevent and therefore more important. Macro fouling is caused by coarse matter of biological or inorganic origin. Such matter enters into the water circuit through the cooling water pumps from sources like the open sea. In closed circuits, like cooling towers, the ingress of macro fouling into the tower basin is possible through open canals or by the wind. Sometimes, parts of the tower internals detach themselves and are carried into the cooling water circuit. Such substances can foul the surfaces of heat exchangers and may cause deterioration of the relevant heat transfer coefficient and they may also create flow blockages, redistribute the flow inside the components, or cause fretting damage. Examples Manmade refuse, Detached internal parts of components, Tools and other foreign objects accidentally left after maintenance, Algae, Mussels, Leaves and these are most often water solutions, but non-aqueous precipitation fouling is also known. Precipitation fouling is a common problem in boilers and heat exchangers operating with hard water. Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, in general, the dependence of the salt solubility on temperature or presence of evaporation will often be the driving force for precipitation fouling. The important distinction is between salts with normal or retrograde dependence of solubility on temperature, the salts with the normal solubility increase their solubility with increasing temperature and thus will foul the cooling surfaces. The salts with inverse or retrograde solubility will foul the heating surfaces, an example of the temperature dependence of solubility is shown in the figure. Calcium sulfate is a common precipitation foulant of heating surfaces due to its retrograde solubility, precipitation fouling can also occur in the absence of heating or vaporization. For example, calcium sulfate decreases it solubility with decreasing pressure and this can lead to precipitation fouling of reservoirs and wells in oil fields, decreasing their productivity with time

4.
Pyridine
–
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group replaced by a nitrogen atom, the pyridine ring occurs in many important compounds, including azines and the vitamins niacin and pyridoxine. Pyridine was discovered in 1849 by the Scottish chemist Thomas Anderson as one of the constituents of bone oil, two years later, Anderson isolated pure pyridine through fractional distillation of the oil. It is a colorless, highly flammable, weakly alkaline, water-soluble liquid with a distinctive, pyridine is used as a precursor to agrochemicals and pharmaceuticals and is also an important solvent and reagent. Pyridine is added to ethanol to make it unsuitable for drinking and it is used in the in vitro synthesis of DNA, in the synthesis of sulfapyridine, antihistaminic drugs tripelennamine and mepyramine, as well as water repellents, bactericides, and herbicides. Some chemical compounds, although not synthesized from pyridine, contain its ring structure and they include B vitamins niacin and pyridoxal, the anti-tuberculosis drug isoniazid, nicotine and other nitrogen-containing plant products. Historically, pyridine was produced from tar and as a byproduct of coal gasification. Pyridine is a liquid that boils at 115.2 °C. Its density,0.9819 g/cm3, is close to that of water, and its index is 1.5093 at a wavelength of 589 nm. Addition of up to 40 mol% of water to pyridine gradually lowers its melting point from −41.6 °C to −65.0 °C, the molecular electric dipole moment is 2.2 debyes. Pyridine is diamagnetic and has a susceptibility of −48.7 × 10−6 cm3·mol−1. The standard enthalpy of formation is 100.2 kJ·mol−1 in the phase and 140.4 kJ·mol−1 in the gas phase. At 25 °C pyridine has a viscosity of 0.88 mPa/s, the enthalpy of vaporization is 35.09 kJ·mol−1 at the boiling point and normal pressure. The enthalpy of fusion is 8.28 kJ·mol−1 at the melting point, pyridine crystallizes in an orthorhombic crystal system with space group Pna21 and lattice parameters a =1752 pm, b =897 pm, c =1135 pm, and 16 formula units per unit cell. For comparison, crystalline benzene is also orthorhombic, with space group Pbca, a =729.2 pm, b =947.1 pm, c =674.2 pm and this difference is partly related to the lower symmetry of the individual pyridine molecule. A trihydrate is known, it crystallizes in an orthorhombic system in the space group Pbca, lattice parameters a =1244 pm, b =1783 pm, c =679 pm. The critical parameters of pyridine are pressure 6.70 MPa, temperature 620 K, the optical absorption spectrum of pyridine in hexane contains three bands at the wavelengths of 195 nm,251 nm and 270 nm. The 1H nuclear magnetic resonance spectrum of pyridine contains three signals with the intensity ratio of 2,1,2 that correspond to the three chemically different protons in the molecule

5.
Salt (chemistry)
–
In chemistry, a salt is an ionic compound that results from the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral and these component ions can be inorganic, such as chloride, or organic, such as acetate, and can be monatomic, such as fluoride, or polyatomic, such as sulfate. There are several varieties of salts, salts that hydrolyze to produce hydroxide ions when dissolved in water are alkali salts, whilst those that hydrolyze to produce hydronium ions in water are acidic salts. Neutral salts are those salts that are neither acidic nor basic, zwitterions contain an anionic centre and a cationic centre in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites, peptides, usually, non-dissolved salts at standard conditions for temperature and pressure are solid, but there are exceptions. Molten salts and solutions containing dissolved salts are called electrolytes, as they are able to conduct electricity. As observed in the cytoplasm of cells, in blood, urine, plant saps and mineral waters, therefore, their salt content is given for the respective ions. Salts can appear to be clear and transparent, opaque, and even metallic, in many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the boundaries, larger crystals tend to be transparent. The color of the salt is due to the electronic structure in the d-orbitals of transition elements or in the conjugated organic dye framework. Different salts can elicit all five basic tastes, e. g. salty, sweet, sour, bitter, and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless and that slow, partial decomposition is usually accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds can be dissolved in water or other similar solvents, the exact combination of ions involved makes each compound have a unique solubility in any solvent. The solubility is dependent on how well each ion interacts with the solvent, for example, all salts of sodium, potassium and ammonium are soluble in water, as are all nitrates and many sulfates – barium sulfate, calcium sulfate and lead sulfate are examples of exceptions. However, ions that bind tightly to each other and form highly stable lattices are less soluble, for example, most carbonate salts are not soluble in water, such as lead carbonate and barium carbonate. Some soluble carbonate salts are, sodium carbonate, potassium carbonate, solid salts do not conduct electricity. Moreover, solutions of salts also conduct electricity, the name of a salt starts with the name of the cation followed by the name of the anion. Salts are often referred to only by the name of the cation or by the name of the anion. g

6.
Jmol
–
Jmol is computer software for molecular modelling chemical structures in 3-dimensions. Jmol returns a 3D representation of a molecule that may be used as a teaching tool and it is written in the programming language Java, so it can run on the operating systems Windows, macOS, Linux, and Unix, if Java is installed. It is free and open-source software released under a GNU Lesser General Public License version 2.0, a standalone application and a software development kit exist that can be integrated into other Java applications, such as Bioclipse and Taverna. A popular feature is an applet that can be integrated into web pages to display molecules in a variety of ways, for example, molecules can be displayed as ball-and-stick models, space-filling models, ribbon diagrams, etc. Jmol supports a range of chemical file formats, including Protein Data Bank, Crystallographic Information File, MDL Molfile. There is also a JavaScript-only version, JSmol, that can be used on computers with no Java, the Jmol applet, among other abilities, offers an alternative to the Chime plug-in, which is no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS9. Jmol requires Java installation and operates on a variety of platforms. For example, Jmol is fully functional in Mozilla Firefox, Internet Explorer, Opera, Google Chrome, fast and Scriptable Molecular Graphics in Web Browsers without Java3D

7.
Chloroform
–
Chloroform, or trichloromethane, is an organic compound with formula CHCl3. It is a colorless, sweet-smelling, dense liquid that is produced on a scale as a precursor to PTFE. It is also a precursor to various refrigerants and it is one of the four chloromethanes and a trihalomethane. The molecule adopts tetrahedral molecular geometry with C3v symmetry, the total global flux of chloroform through the environment is approximately 7005660000000000000♠660000 tonnes per year, and about 90% of emissions are natural in origin. Many kinds of seaweed produce chloroform, and fungi are believed to produce chloroform in soil and its half-life in air ranges from 55 to 620 days. Biodegradation in water and soil is slow, chloroform does not significantly bioaccumulate in aquatic organisms. Justus von Liebig carried out the cleavage of chloral. Eugène Soubeiran obtained the compound by the action of chlorine bleach on both ethanol and acetone, in 1834, French chemist Jean-Baptiste Dumas determined chloroforms empirical formula and named it. In 1835, Dumas prepared the substance by the cleavage of trichloroacetic acid. Regnault prepared chloroform by chlorination of chloromethane, in 1842 Dr Robert Mortimer Glover in London discovered the anaesthetic qualities of chloroform on laboratory animals. In 1847, Scottish obstetrician James Y. Simpson was the first to demonstrate the properties of chloroform on humans. By the 1850s, chloroform was being produced on a basis by using the Liebig procedure. Today, chloroform — along with dichloromethane — is prepared exclusively, in industry, chloroform is produced by heating a mixture of chlorine and either chloromethane or methane. CDCl3 is a solvent used in NMR spectroscopy. Deuterochloroform is produced by the reaction, the reaction of acetone with sodium hypochlorite or calcium hypochlorite. The haloform process is now obsolete for the production of ordinary chloroform, deuterochloroform can also be prepared by the reaction of sodium deuteroxide with chloral hydrate, or from ordinary chloroform. The haloform reaction can also occur inadvertently in domestic settings, bleaching with hypochlorite generates halogenated compounds in side reactions, chloroform is the main byproduct. Chlorodifluoromethane is then converted into tetrafluoroethylene, the precursor to Teflon

8.
Benzene
–
Benzene is an important organic chemical compound with the chemical formula C6H6. The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen atom attached to each, because it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. Benzene is a constituent of crude oil and is one of the elementary petrochemicals. Because of the cyclic continuous pi bond between the atoms, benzene is classed as an aromatic hydrocarbon, the second -annulene. Benzene is a colorless and highly flammable liquid with a sweet smell and it is used primarily as a precursor to the manufacture of chemicals with more complex structure, such as ethylbenzene and cumene, of which billions of kilograms are produced. Because benzene has a high number, it is an important component of gasoline. Because benzene is a carcinogen, most non-industrial applications have been limited. The word benzene derives historically from gum benzoin, a resin known to European pharmacists. An acidic material was derived from benzoin by sublimation, and named flowers of benzoin, the hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene. Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, in 1833, Eilhard Mitscherlich produced it by distilling benzoic acid and lime. He gave the compound the name benzin, in 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first industrial-scale production of benzene, gradually, the sense developed among chemists that a number of substances were chemically related to benzene, comprising a diverse chemical family. In 1855, Hofmann used the word aromatic to designate this family relationship, in 1997, benzene was detected in deep space. The empirical formula for benzene was known, but its highly polyunsaturated structure. In 1865, the German chemist Friedrich August Kekulé published a paper in French suggesting that the structure contained a ring of six carbon atoms with alternating single and double bonds, the next year he published a much longer paper in German on the same subject. Kekulés symmetrical ring could explain these facts, as well as benzenes 1,1 carbon-hydrogen ratio. Here Kekulé spoke of the creation of the theory and he said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail. This vision, he said, came to him years of studying the nature of carbon-carbon bonds

9.
Dichloromethane
–
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with organic solvents. One of the most well-known applications of dichloromethane is in the drinking bird heat engine, natural sources of dichloromethane include oceanic sources, macroalgae, wetlands, and volcanoes. However, the majority of dichloromethane in the environment is the result of industrial emissions, DCM is produced by treating either chloromethane or methane with chlorine gas at 400–500 °C. At these temperatures, both methane and chloromethane undergo a series of reactions producing progressively more chlorinated products, in this way, an estimated 400,000 tons were produced in the US, Europe, and Japan in 1993. These compounds are separated by distillation, DCM was first prepared in 1839 by the French chemist Henri Victor Regnault, who isolated it from a mixture of chloromethane and chlorine that had been exposed to sunlight. DCMs volatility and ability to dissolve a wide range of organic compounds makes it a useful solvent for chemical processes. It is widely used as a paint stripper and a degreaser, in the food industry, it has been used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings. Its volatility has led to its use as an aerosol spray propellant, the chemical compounds low boiling point allows the chemical to function in a heat engine that can extract mechanical energy from small temperature differences. An example of a DCM heat engine is the drinking bird, the toy works at room temperature. For example, it is used to seal the casing of electric meters, often sold as a main component of plastic welding adhesives, it is also used extensively by model building hobbyists for joining plastic components together. It is commonly referred to as Di-clo and it is used in the garment printing industry for removal of heat-sealed garment transfers, and its volatility is exploited in novelty items, bubble lights and jukebox displays. DCM is the least toxic of the simple chlorohydrocarbons, but it is not without health risks and it can also be absorbed through the skin. More severe consequences can include suffocation, loss of consciousness, coma, DCM is also metabolized by the body to carbon monoxide potentially leading to carbon monoxide poisoning. Acute exposure by inhalation has resulted in optic neuropathy and hepatitis, prolonged skin contact can result in DCM dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns. It may be carcinogenic, as it has linked to cancer of the lungs, liver. Other animal studies showed breast cancer and salivary gland cancer, research is not yet clear as to what levels may be carcinogenic

10.
Tetrahydrofuran
–
Tetrahydrofuran, whose preferred IUPAC name was changed in 2013 to oxolane, is an organic compound with the formula 4O. The compound is classified as heterocyclic compound, specifically a cyclic ether and it is a colorless, water-miscible organic liquid with low viscosity. It is mainly used as a precursor to polymers, being polar and having a wide liquid range, THF is a versatile solvent. About 200,000 tonnes of tetrahydrofuran are produced annually, the most widely used industrial process involves the acid-catalyzed dehydration of 1, 4-butanediol. Ashland/ISP is one the biggest producers of this chemical route, the method is similar to the production of diethyl ether from ethanol. The butanediol is derived from condensation of acetylene with formaldehyde followed by hydrogenation, duPont developed a process for producing THF by oxidizing n-butane to crude maleic anhydride, followed by catalytic hydrogenation. A third major industrial route entails hydroformylation of allyl alcohol followed by hydrogenation to the butanediol, THF can also be synthesized by catalytic hydrogenation of furan. Certain sugars can be converted to THF, although this method is not widely practiced, furan is thus derivable from renewable resources. The other main application of THF is as a solvent for polyvinyl chloride. It is a solvent with a dielectric constant of 7.6. It is a polar solvent and can dissolve a wide range of nonpolar and polar chemical compounds. THF is water-miscible and can form solid clathrate hydrate structures with water at low temperatures, in the laboratory, THF is a popular solvent when its water miscibility is not an issue. It is more basic than diethyl ether and forms complexes with Li+, Mg2+. It is a solvent for hydroboration reactions and for organometallic compounds such as organolithium. Although similar to diethyl ether, THF is a stronger base, commercial THF contains substantial water that must be removed for sensitive operations, e. g. those involving organometallic compounds. Although THF is traditionally dried by distillation from an aggressive desiccant, aqueous THF augments the hydrolysis of glycans from biomass and dissolves the majority of biomass lignin making it a suitable solvent for biomass pretreatment. THF is often used in polymer science, for example, it can be used to dissolve polymers prior to determining their molecular mass using gel permeation chromatography. THF dissolves PVC as well, and thus it is the ingredient in PVC adhesives

An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative …

Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion. Red: oxygen, black: carbon, white: hydrogen.